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Re: (lack of) message header field ordering

2005-03-14 07:00:21

I think a simpler approach to this problem would be to
encourage sites to append locally defined trace information
(e.g. anti-spam scores) to the start of the header instead of
the end, so that it appears interleaved with the Received:
fields. This makes it much clearer when a field was added. A
fair amount of software already does this, e.g. Delivered-To:
and X-Sieve: fields. It also happens to agree with the logic
of DomainKeys signatures :-)

As pointed out earlier, doing this risks getting a much higher
spam probability score from software that doesn't know about
your particular new headers and hence treats them as evidence of
header-tampering.

As I said, they have to deal with this already.

The advantages of separating MTA-provided in-transit information from
message body ("header") information are pretty obvious. To move that
way, we would then need to address the question of balancing ease of
converting to and from the new mechanism and format against additional
power and flexibility from new formats.

I think that it will be easiest to enhance the semantics of the existing
header format than to add a new mechanism for separate trace information.
In practice you can draw an imaginary line through the header immediately
after the textually-last Received: field; everything before the line will
relate closely to the path that the message took (trace fields, resent-*
fields, locally-defined fields); what comes after the line will be mostly
user-related, but it will be jumbled with trace-like stuff such as List-*
fields and spam score fields. This is a practical distinction based on
observation rather than a formal one, so it's not going to be perfect. If
you can encourage implementations to add their trace-like stuff to the
start of the header then this observed distinction will become gradually
more and more useful. No need for significant new functionality, no loss
of functionality caused by MTAs in a message's path that don't understand
the distinction. It does depend on a reasonable expectation of minimal
re-ordering, but that is true almost all the time.

Tony.
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